The present invention relates to machines for weighing and classifying tablets, particularly at a high rate of speed. The invention is related to U.S. Pat. No. 5,135,113, issued Aug. 4, 1992, which utilizes the basic principle of capacitance, wherein the weight of a tablet can be determined within precise limits by passing the tablet through a capacitance sensor, and measuring the change in capacitance which results from the presence of the tablet in the sensor. The present invention is particularly related to the mechanism for reliably feeding a stream of tablets in proper orientation and without extraneous particles to the capacitance sensor.
In the prior art, high-speed machines have been developed for classifying pharmaceutical capsules, as in U.S. Pat. No. 4,223,751, issued Sep. 23, 1980, and owned by the assignee of the present invention. A further and related patent is found in U.S. Pat. No. 4,402,412, issued Sep. 6, 1983. In both of the foregoing patents the weight of a capsule in a stream of capsules is determined by passing the moving capsule through a fixed capacitive sensor arranged in tubular form, wherein the resultant change in capacitance provides a measure of capsule weight. A deflector mechanism is provided downstream in the path of capsule travel, and the capsules which are measured outside of certain weight parameters are deflected from the path of travel into a reject bin. Capsules which measure within the desired weight range are permitted to continue in the path of travel to be received in a second bin.
The basic capacitance principles of the foregoing patents are utilized in U.S. Pat. No. 5,135,113 in a constructional form to accommodate the different problems which arise in connection with measuring the weight of tablets versus measuring the weight of capsules. Whereas a capsule is constructed of uniform form, having a symmetry about a longitudinal axis, a tablet tends to be of nonuniform shape and of varying size. Capsules are symmetrical about a longitudinal axis and are therefore amenable to propagation through a tubular passage, whereas tablets are of somewhat nonuniform disk shapes, having a predetermined diameter and varying thickness dimensions. When tablets are propagated through a tube they tend to tumble and jam up in the flow path, thereby rendering devices of the type disclosed in the earlier patents useless for weighing and classifying tablets, and thereby inspiring the invention of the '113 patent. Tablets usually do not have flat top and bottom surfaces, but have surfaces with varying degrees of roundness. Tablets are much more prone to chip and break, and extraneous pieces of tablets frequently get mixed into the tablet stream and are conveyed along with good tablets.
The problem of conveying a stream of tablets into the sensing apparatus of the '113 patent, wherein the tablets in the stream are all properly oriented and the number of broken and extraneous tablet particles are held to an absolute minimum, is solved with the present invention.